Install libsdl1.2-dev inside container

Make sure the following check from poky/meta/classes/sanity.bbclass passes:
```
if "libsdl-native" in assume_provided:
     if not check_app_exists("sdl-config", d):
        status.addresult("libsdl-native is set to be ASSUME_PROVIDED but sdl-config can't be found in PATH. Please either install it, or configure qemu not to require sdl.")
```

Signed-off-by: Gianpaolo Macario <gianpaolo_macario@mentor.com>